975e7e405a
dirty_memory_manager tracks two quantities about memtable memory usage: "real" and "unspooled" memory usage. "real" is the total memory usage (sum of `occupancy().total_space()`) by all memtable LSA regions, plus a upper-bound estimate of the size of memtable data which has already moved to the cache region but isn't evictable (merged into the cache) yet. "unspooled" is the difference between total memory usage by all memtable LSA regions, and the total flushed memory (sum of `_flushed_memory`) of memtables. dirty_memory_manager controls the shares of compaction and/or blocks writes when these quantities cross various thresholds. "Total flushed memory" isn't a well defined notion, since the actual consumption of memory by the same data can vary over time due to LSA compactions, and even the data present in memtable can change over the course of the flush due to removals of outdated MVCC versions. So `_flushed_memory` is merely an approximation computed by `flush_reader` based on the data passing through it. This approximation is supposed to be a conservative lower bound. In particular, `_flushed_memory` should be not greater than `occupancy().total_space()`. Otherwise, for example, "unspooled" memory could become negative (and/or wrap around) and weird things could happen. There is an assertion in ~flush_memory_accounter which checks that `_flushed_memory < occupancy().total_space()` at the end of flush. But it can fail. Without additional treatment, the memtable reader sometimes emits data which is already deleted. (In particular, it emites rows covered by a partition tombstone in a newer MVCC version.) This data is seen `flush_reader` and accounted in `_flushed_memory`. But this data can be garbage-collected by the mutation_cleaner later during the flush and decrease `total_memory` below `_flushed_memory`. There is a piece of code in mutation_cleaner intended to prevent that. If `total_memory` decreases during a `mutation_cleaner` run, `_flushed_memory` is lowered by the same amount, just to preserve the asserted property. (This could also make `_flushed_memory` quite inaccurate, but that's considered acceptable). But that only works if `total_memory` is decreased during that run. It doesn't work if the `total_memory` decrease (enabled by the new allocator holes made by `mutation_cleaner`'s garbage collection work) happens asynchronously (due to memory reclaim for whatever reason) after the run. This patch fixes that by tracking the decreases of `total_memory` closer to the source. Instead of relying on `mutation_cleaner` to notify the memtable if it lowers `total_memory`, the memtable itself listens for notifications about LSA segment deallocations. It keeps `_flushed_memory` equal to the reader's estimate of flushed memory decreased by the change in `total_memory` since the beginning of flush (if it was positive), and it keeps the amount of "spooled" memory reported to the `dirty_memory_manager` at `max(0, _flushed_memory)`.
Scylla
What is Scylla?
Scylla is the real-time big data database that is API-compatible with Apache Cassandra and Amazon DynamoDB. Scylla embraces a shared-nothing approach that increases throughput and storage capacity to realize order-of-magnitude performance improvements and reduce hardware costs.
For more information, please see the ScyllaDB web site.
Build Prerequisites
Scylla is fairly fussy about its build environment, requiring very recent versions of the C++23 compiler and of many libraries to build. The document HACKING.md includes detailed information on building and developing Scylla, but to get Scylla building quickly on (almost) any build machine, Scylla offers a frozen toolchain, This is a pre-configured Docker image which includes recent versions of all the required compilers, libraries and build tools. Using the frozen toolchain allows you to avoid changing anything in your build machine to meet Scylla's requirements - you just need to meet the frozen toolchain's prerequisites (mostly, Docker or Podman being available).
Building Scylla
Building Scylla with the frozen toolchain dbuild is as easy as:
$ git submodule update --init --force --recursive
$ ./tools/toolchain/dbuild ./configure.py
$ ./tools/toolchain/dbuild ninja build/release/scylla
For further information, please see:
- Developer documentation for more information on building Scylla.
- Build documentation on how to build Scylla binaries, tests, and packages.
- Docker image build documentation for information on how to build Docker images.
Running Scylla
To start Scylla server, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --workdir tmp --smp 1 --developer-mode 1
This will start a Scylla node with one CPU core allocated to it and data files stored in the tmp directory.
The --developer-mode is needed to disable the various checks Scylla performs at startup to ensure the machine is configured for maximum performance (not relevant on development workstations).
Please note that you need to run Scylla with dbuild if you built it with the frozen toolchain.
For more run options, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --help
Testing
See test.py manual.
Scylla APIs and compatibility
By default, Scylla is compatible with Apache Cassandra and its API - CQL. There is also support for the API of Amazon DynamoDB™, which needs to be enabled and configured in order to be used. For more information on how to enable the DynamoDB™ API in Scylla, and the current compatibility of this feature as well as Scylla-specific extensions, see Alternator and Getting started with Alternator.
Documentation
Documentation can be found here. Seastar documentation can be found here. User documentation can be found here.
Training
Training material and online courses can be found at Scylla University. The courses are free, self-paced and include hands-on examples. They cover a variety of topics including Scylla data modeling, administration, architecture, basic NoSQL concepts, using drivers for application development, Scylla setup, failover, compactions, multi-datacenters and how Scylla integrates with third-party applications.
Contributing to Scylla
If you want to report a bug or submit a pull request or a patch, please read the contribution guidelines.
If you are a developer working on Scylla, please read the developer guidelines.
Contact
- The community forum and Slack channel are for users to discuss configuration, management, and operations of ScyllaDB.
- The developers mailing list is for developers and people interested in following the development of ScyllaDB to discuss technical topics.